The field of the disclosure relates generally to digital transmission systems, and more particularly, to wired, wireless, and optical digital transmission systems.
Conventional digital transmission systems typically include both linear and non-linear distortion. However, for the purposes of the following discussion, use of the term “distortion” is generally intended to refer to nonlinear distortion. Some digital transmission systems utilize lasers or semiconductor lasers for data transmission. Semiconductors lasers, however, are known to “chirp.” A chirp refers to a change of the wavelength of a laser, typically caused by laser instability. Chirps can indicate an increase of a signal frequency over time, sometimes referred to as an “up-chirp,” or a decrease, sometimes referred to as a “down-chirp.” Thus, as the signal intensity increases, the wavelength changes accordingly.
That is, chirp often refers to the frequency shift of laser sources when the laser intensity is changed. In an optical system with chromatic dispersion, such frequency shifts may cause changes in propagation speed for different parts of an amplitude-modulated signal. Where the laser source includes a semiconductor laser diode, the refractive index of the semiconductor material changes based on the current passed through the material. Optical chirps are produced as the laser is modulated, and the current density changes accordingly. More specifically, nonlinear distortion is created when a chirping light signal is passed through a long fiber optic cable having chromatic dispersion. This nonlinear distortion causes inter-symbol interference (ISI), which will limit the maximum distance, the maximum current drive, and the maximum bandwidth of which the system is capable.
Some conventional transmission technology utilizes Data Over Cable Service Interface Specification (DOCSIS) or DOCSIS version 3.1 (DOCSIS 3.1) technology and orthogonal frequency division multiplexing (OFDM) and/or orthogonal frequency division multiple access (OFDMA) modulation. Conventional cable plants may include hundreds of transmitters operating in burst mode, and are configured to transmit OFDM and OFDMA signals through wired and wireless channels. Systems and methods for measuring nonlinear distortion are described in U.S. Pat. Nos. 9,209,863, 9,225,387, and 9,590,696, the disclosures of which are incorporated by reference herein.
FIG. 1A is a graphical illustration depicting a conventional sampled signal 100. Signal 100 diagrams a sine wave of a series 102 of individual amplitude samples over time. FIG. 1B is a graphical illustration depicting a conventional sampled signal 104, which is an example of sampled signal 100, FIG. 1A, having chromatic dispersion and exhibiting a chirp. More specifically, signal 104 illustrates an example of signal 100 after having been transmitted using a laser exhibiting an optical frequency that varies with current drive, and after having gone through a fiber optic cable that produces chromatic dispersion. Signal 104 similarly diagrams a sine wave of a series 106 of individual amplitude samples over time. As individual values of series 106 increase, a delay 108 also increases, thus further distorting the waveform of signal 104.
Accordingly, a comparison of signals 100 and 104 illustrates the effect of a chirp, plus chromatic dispersion, on a sine wave. The combination of chirp and chromatic dispersion adds nonlinear distortion. In single mode glass fiber, chromatic dispersion, which is similar to group delay from filters, results from different frequencies (optical wavelengths) traveling at different speeds down the fiber optic cable.